1. Field of the Invention
The present invention generally relates to the field of integrated circuits. In particular, the present invention is directed to a programmable electronic antifuse and circuits made therewith.
2. Background of the Invention
Programmable fuses and antifuses are devices useful in a variety of integrated circuits (ICs). Programmable fuses and antifuses allow IC designers to “personalize,” or custom configure, various circuits to provide the respective circuits with desired functionality and/or reconfigure the circuits to bypass one or more defective elements or utilize redundant elements or circuits to replace defective elements or circuits. A programmable fuse is a device that is closed in its unprogrammed state and open when programmed. Generally, a programmable fuse comprises a fusible conductive link that is broken during programming so that the conductive link no longer closes the circuit.
Programmable fuses are generally of either a laser programmable type or an electronically programmable type. In both types, the fusible conductive link is broken by heating the link sufficiently so as to cause the link to melt. In the laser programmable type of fuse, a laser provides the energy that melts the conductive link. In the electronically programmable type, a relatively large current is passed through the conductive link such that the resistive heating of the link causes the link to melt. Laser programmable fuses are not suitable in many ICs due to the fact that it is often desirable to program the fuses after the IC has been encapsulated and packaged. This is so because laser programmable fuses must be exposed so that a laser beam can be shone upon them in order to melt them. Conventional electronically programmable fuses are generally not desirable for today's CMOS-based ICs due to the high programming voltages and currents required and the non-CMOS fabrication techniques needed to make some fuses that add to the overall cost and complexity of making the ICs.
A programmable antifuse, on the other hand, is a device that is open in its unprogrammed state and closed when programmed. Conventionally, an antifuse comprises two conductive regions separated by an insulating region that electrically insulates the two conductive regions from one another. In its unprogrammed state, the antifuse generally acts as a capacitor, with no current passing from one conductive region to the other. When programmed, however, the insulating ability of the insulating region is at least partially destroyed, allowing current to flow between the two conductive regions. Typically, programming an antifuse results in the formation of a conductive filament that extends within the insulating region between the two conductive regions.
To date, several types of programmable antifuse devices have been proposed and used in various ICs. Generally, all of these types are electronically programmable, i.e., programmable by charging the conductive regions of the antifuses with a voltage high enough to cause the insulating region between these regions to break down sufficiently to cause it to become at least partially conductive. One type of programmable antifuse is a metal-oxide-metal antifuse. This type of antifuse structure is not desirable in CMOS-based ICs because of the non-standard processing techniques needed to fabricate the metal conductive portions of the antifuse.
Another type of conventional programmable antifuse is a polysilicon antifuse having its insulating region made of polysilicon. Although this type of antifuse is compatible with CMOS processing techniques, existing polysilicon antifuses include fusible links that typically require 10 mA to 15 mA of programming current at a voltage higher than needed during normal operation of the IC containing the antifuses. Consequently, the wiring that carries the programming current for such polysilicon antifuses needs to be robust. This increases the area needed for the antifuses and associated wiring. In addition, existing cobalt silicide polysilicon antifuses undergo only a relatively small change in resistance from its unprogrammed state to its programmed state. Therefore, the sensing circuits needed to sense such small resistance changes must be very robust and complex. Such sensing circuits require many elements, and their resistive trip points may need to be adjusted frequently to manage large tails in resistive distributions.
More recently, antifuse designers have utilized current oxide technology in an attempt to improve the density of antifuse devices within ICs and to reduce programming current by at least two orders of magnitude over present polysilicon antifuse technology. In addition to reducing the size of programming wiring, an additional benefit of achieving such low programming currents, e.g., in the microamp regime, is the ability to program hundreds or more antifuses at once, thereby reducing the time needed to program all of the antifuses. Recent efforts utilizing trench or thin-oxide field-effect transistor (FET) structures have had very good results in terms of programming current. Such antifuses have required as little as 1 μA. However, the resistances of these antifuses in their programmed state are typically in the mega-ohm regime. Techniques for sensing such high-resistance fuses have been shown in the prior art, e.g., in U.S. Pat. No. 6,426,668, entitled “Imbalanced Sense Amplifier Fuse Detection Circuit,” owned by the assignee of the present invention, among other publications.
A fundamental problem with antifuses having conducting regions made with semiconductor materials, e.g., when the antifuses are made from CMOS FET structures, is that the conductive filament resulting from programming has a highly non-linear resistance. This characteristic of such antifuses is due to the migration of dopant atoms from the gate and channel of the FET into the gate oxide insulating layer between the gate and channel. Consequently, the resistance is low at high voltages, but very high at the low voltages at which the antifuse will operate once programmed. The result of the very high resistances of the programmed state is that the complex sensing circuits mentioned above must be used to sense the programmed state of the antifuse. While such sensing circuits exist and are effective in discriminating small differences between unprogrammed and programmed resistances, they typically are relatively large circuits having a dynamic design sensitive to noise, voltage variations and process variations. These large sensing circuits result in lower antifuse density and increased circuit complexity.
In view of the foregoing, a need exists for an antifuse technology having a several order of magnitude change between unprogrammed and programmed resistances that can be reliably sensed by a simple sensing circuit, and which can be readily integrated into conventional CMOS processing.